The present invention relates to a monocrystal producing technique, and in particular to a producing device and a producing process for producing a monocrystal by a pulling-down method, and a monocrystal.
In recent years, monocrystals of oxides such as lithium tantalate LiTaO3 (referred to as LT hereinafter), lithium niobate LiNbO3 (referred to as LN hereinafter), lithium tetraborate Li2B4O7 (referred to as LBO hereinafter), and langasite La3Ga5SiO14 (referred to as LGS hereinafter) have been used to produce various surface acoustic wave devices. These monocrystals are piezoelectric crystals having a large electromechanical coupling coefficient than that of a quartz crystal substrate. LBO and LGS have a cut angle of a zero temperature coefficient. Therefore, if these monocrystals are used for a surface acoustic wave device, terminals such as a portable telephone become small-sized and come to have high functions. In crystals of LT and LN, the ratio of Li to Ta or the ratio of Li to Nb stoichiometrically becomes 1:1. Crystals having such a composition are suitable for optical materials since they have in their lattices no defect or gap to become an ideal crystal structure and they have a constant refractive index in the crystals thereof to generate no diffuse reflection.
The methods for growing the above-mentioned monocrystals are roughly classified into the following three methods. That is, the methods are the Czochralski method (CZ method), the vertical Bridgman method (VB method) and a pulling-down method.
As shown in FIG. 6, the Czochralski method (CZ method, rotation pulling method) is the method of putting a raw material to be crystallized into a platinum crucible 41, heating the raw material to the melting point thereof or higher in an electric furnace 42 to be melted, immersing the lower end of a seed crystal 44 in a rod form into the resultant 43, and pulling the crystal with slow rotation so as to grow a crystal 45 from the lower end of the seed crystal 44.
As shown in FIG. 7, the vertical Bridgman method is the method of putting a raw material to be crystallized into a platinum crucible 51, heating the raw material to the melting point thereof or higher in an electric furnace 52 to be melted, putting a plate-form seed crystal 53 into the platinum crucible 51 from one end thereof, and slowly moving the platinum crucible 51, with keeping the side of the seed crystal 53 ahead, from the side of high temperature to the side of low temperature in the state that temperature gradient is generated inside the electric furnace 52, so as to grow a crystal successively from the side of the seed crystal 53.
The pulling-down method is a monocrystal growing method published in a document (Journal of the Ceramic Society of Japan 105[7] 1997) by one of the inventors of the present application. As shown in FIG. 8, this method is the method of putting a polycrystal raw material into a platinum crucible 61 having a fine hole 610a in its bottom, arranging this platinum crucible 61 at the position where temperature gradient is steepest inside an electrical furnace 62 whose upper side is kept over the melting point of the raw material and whose lower side is kept below the melting point thereof to melt the raw material, and pulling down a seed crystal 63 with rotation in the state that the upper end of the seed crystal 63 in a rod form is brought into contact with the raw material melt that has flown out from the fine hole 61a of the platinum crucible 61 by gravity. According to this pulling-down method, the raw material melt is kept between the platinum crucible 61 and the seed crystal 63, using both of wettability of the raw material melt to the crucible 61, this melt being a melt that has leaked out from the fine hole 61a of the bottom of the platinum crucible 61, and surface tensile thereof, so as to grow a crystal.
In general, monocrystals such as LN, LT and LGS are grown by the rotation pulling method (CZ method), and LBO is mainly grown by the vertical Bridgman method (VB method). LBO can be however grown by the CZ method.
However, conventional monocrystal growing methods, which are represented by the CZ method, have the following problems.
In connection with the melting points of raw materials, it is general that a platinum crucible is necessary for the growth of LN and an iridium crucible is necessary for the growth of LT and LGS. In connection with crystal size, a crucible of about 4 kg and a crucible of about 5 kg are necessary for the growth of, e.g., a crystal of 3 inches in diameter and a crystal of 4 inches in diameter, respectively. If an after-heater is used to keep growing temperature constant, a noble metal, such as platinum and iridium, of 1-2 kg becomes necessary. Since such a noble metal is used in a large amount, a large burden is imposed from the viewpoint of costs.
The conventional methods are in a so-called batch manner, in which it is necessary that not only a monocrystal to be pulled from a crucible and grown but also a considerably excess monocrystal are melted in the crucible and the total amount thereof is kept over the melting point. Therefore, there are limitations in enlarging the diameter of a pulled crystal and making the crystal long. Furthermore, electric power consumed with the heater and the like increases largely as the crystal becomes larger.
Crystals such as LN and LT have a wide solid solution area. Since their chemical composition is different from its congruent melt composition, their composition easily changes between the initial and final periods of the growing of the crystals. If, for example, temperature and the molar ratio (%) of lithium oxide (LiO2) are expressed by a vertical axis and a horizontal axis, respectively, the state diagram (phase diagram) of LT is as shown in FIG. 9. When the monocrystal having varied compositions are used to make SAW devices, their propagation speeds and piezoelectric constants are scattered, so as to result in a drop in the yield of products.
In the CZ method, reacting treatments and high temperature treatments such as mixing of raw materials of tantalum pentaoxide Ta2O5 and lithium carbonate Li2CO3, sintering, pulverizing and press are conducted as a treatment before a raw material is charged into a crucible. Therefore, the composition changes in the step of preparing the raw material by evaporation of lithium oxide Li2O or the like, which has a high vapor pressure. Besides, the composition of the resultant crystal changes by evaporation of a specific substance in the crystal growing step.
In the CZ method, a monocrystal rod is grown by the steps of sowing seeds, producing a shoulder portion, and then growing a body portion. However, it takes a long time to grow the body portion. Moreover, in order to obtain the body portions having a little dispersion of their diameters, a high-priced ADC (Automatic Diameter Control) device is necessary, so that costs for the production thereof become high.
As can also be. understood in FIG. 9, crystals such as LN and LT are crystallized into their congruent melt composition at T1, which is the highest temperature in the case that the liquid phase is changed into the solid phase. When the melt of Li and Nb is put into a crucible and a seed crystal is pulled upon the growth of a crystal by the CZ method, the initial growth of the crystal (the crystal containing Nb whose amount is larger than that of Li) advances in the state of its congruent composition, which causes easy crystallization. However, Li and Nb are beforehand mixed at a ratio of 1 to 1 in the crucible, so that the amount of Nb becomes smaller than that of Li in the melt as the growth of the crystal advances. As a result, a crystal having a content of Li larger than the Li content in the congruent composition is grown when crystallization advances slowly. That is, regions having different compositions are generated in a monocrystal.
Accordingly, in the CZ method a melt is beforehand made up so as to have a congruent composition, and the CZ method is generally used to grow a congruent crystal. In this case, however, the melt does not always have a uniform composition. Thus, the composition is easily scattered. This is also the same in the Bridgman method. So far as all of necessary materials need to be beforehand put into a crucible, the scattering cannot be avoided. Therefore, it is difficult to grow crystals having compositions other than the congruent composition, that is, incongruent compositions (including a stoichiometrical composition).
The congruent compositions of LN and LT are as follows:
Li/(Li+Nb)xc3x97100≈48%,
and
Li/(Li+Ta)xc3x97100≈48%.
Therefore, in order to obtain a crystal having an incongruent composition (the component ratio of Li is from 48.5 to 50.0% in the case of, e.g., LN or LT), it has been inevitable up to the present to use other methods. For example, a monocrystal growing method of xe2x80x9cthe double crucible methodxe2x80x9d is known. As is well known, however, the diameter of a crystal that can be grown is small and the limitation thereof is about 1 inch. In the fields in which LN and LT are used as piezoelectric materials or optical materials of a SAW device or the like, a larger monocrystal has been desired from the viewpoint of productivity. It has been desired to develop a crystal growing method making it possible to grow a large monocrystal having a stable composition and a diameter of more than 1 inch at a low price.
The vertical Bridgman method is a standard method for growing an LBO monocrystal at present. It is necessary to prepare a platinum crucible newly whenever an LBO crystal is grown at a time. Thus, a problem arises that costs for production thereof become high.
In the light of the above-mentioned situations, the present invention has been made. A problem that should be solved is to provide a monocrystal producing device and a monocrystal producing process that make it possible to produce a monocrystal having a stable composition and having a large diameter and a long size at a low price, and provide an LN monocrystal, an LT monocrystal or the like that has a diameter of more than 1 inch and a stable composition.
The monocrystal producing device of the invention recited in claim 1 is a monocrystal producing device for growing a crystal by arranging a crucible for melting a raw material in an electric furnace, keeping the crucible at a temperature not less than the melting point of the raw material, and pulling down and simultaneously rotating a seed crystal in the state that the upper end portion of the seed crystal is brought into contact with a raw material melt that has leaked out from a fine hole made at the bottom portion of the crucible, characterized by comprising a powdery raw material supplying means for introducing a powdery raw material into the crucible from the above, and a premelt plate for receiving the powdery raw material from this powdery raw material supplying means to be melted and subsequently introducing the melt into a melt-collecting portion of the crucible.
According to the monocrystal producing device made as above, the powdery raw material is supplied onto the premelt plate with the powdery raw material supplying means and then the powdery raw material is melted on the premelt plate to generate the raw material melt and introduce the raw material melt into the melt-collecting portion of the crucible to keep the outflow amount of the raw material melt from the fine hole of the bottom of the crucible substantially constant. In this way, a crystal can be grown while the raw material melt is continuously supplied into the crucible. It is therefore possible to obtain easily a monocrystal having a large diameter and a long size. The process for growing the crystal from the powdery raw material can be continuously performed so that the composition of the obtained monocrystal becomes stable. High-priced constituting elements such as the platinum crucible can be used semipermanently after only initial investment. Thus, producing costs can be made low.
The monocrystal producing device of the invention recited in claim 2 is characterized in that the powdery raw material supplying means in claim 1 comprises a powdery raw material tank for receiving the powdery raw material, a dry air introducing means for introducing a dry air into the powdery raw material inside this powdery raw material tank, and a raw material transferring means for transferring the powdery raw material from this powdery raw material tank onto the premelt plate.
According to the monocrystal producing device made as above, by introducing the dry air into the powdery raw material to remove the moisture of the raw material powder, it is possible to prevent condensation of the raw material powder based on the moisture and supply stably the powdery raw material having a constant ratio of the components onto the premelt plate.
The monocrystal producing device of the invention recited in claim 3 is characterized in that the premelt plate in claim 2 is, together with the crucible, arranged inside the electric furnace, and the raw material transferring means comprises a transferring tube whose one end is connected to the powdery raw material tank and whose other end is inserted into the electric furnace to transfer the powdery raw material onto the premelt plate, and a cooling means for cooling this transferring tube from the outside.
According to the monocrystal producing device made as above, the structure of the device can be made simple since the crucible and the premelt plate can be heated with the single electric furnace. Moreover, by cooling the transferring tube for transferring the powdery raw material from the outside of the electric furnace onto the premelt plate inside the electric furnace, it is possible to prevent the melting of the powdery raw material in the middle of the transferring tube and the filling in the transferring tube.
The monocrystal producing device of the invention recited in claim 4 is a monocrystal producing device for growing a crystal by arranging a crucible for melting a raw material in an electric furnace, keeping the crucible at a temperature not less than the melting point of the raw material, and pulling down and simultaneously rotating a seed crystal in the state that the upper end portion of the seed crystal is brought into contact with a raw material melt that has leaked out from a fine hole made at the bottom portion of the crucible, characterized by comprising a raw material melting tank for melting a powdery raw material (a raw material in a powdery state) to generate the raw material melt, a powdery raw material supplying means for introducing the powdery raw material into this raw material melting tank, and a raw material melt introducing means for introducing the raw material melt inside the raw material melting tank into the crucible.
According to the monocrystal producing device made as above, the powdery raw material is supplied to the raw material melting tank with the powdery raw material supplying means and the raw material is melted in the raw material melting tank to generate the raw material melt. This raw material melt is introduced into the crucible with the raw material melt introducing means. In this way, the crystal can be grown while the raw material melt is supplied into the crucible. Therefore, the crystal can be grown while the amount of the melt in the crucible from the start of the crystal growth to the end thereof is kept substantially constant to keep the outflow amount of the raw material melt from the fine hole of the bottom portion for the crucible.
The monocrystal producing device of the invention is characterized in that the powdery raw material supplying means in claim 4 comprises a powdery raw material tank for receiving the powdery raw material, a dry air introducing means for introducing a dry air into the powdery raw material inside this powdery raw material tank, and a raw material transferring means for transferring the powdery raw material from this powdery raw material tank into the raw material melting tank.
According to the monocrystal producing device made as above, by introducing the dry air into the powdery raw material to remove the moisture of the raw material powder, it is possible to prevent condensation of the raw material based on the moisture and supply the powdery raw material having a constant composition ratio stably.
The monocrystal producing device of the invention is characterized in that the raw material melting tank is, together with the crucible, arranged inside the electric furnace, and the raw material transferring means comprises a transferring tube whose one end is connected to the powdery raw material tank and whose other end is inserted into the electric furnace to transfer the powdery raw material onto the raw material melting tank, and a cooling means for cooling this transferring tube from the outside.
According to the monocrystal producing device made as above, the structure of the device can be made simple since the crucible and the raw material melting tank can be heated with the single electric furnace. Moreover, by cooling the transferring tube for transferring the powdery raw material from the outside of the electric furnace into the raw material melting tank inside the electric furnace, it is possible to prevent the melting of the powdery raw material in the middle of the transferring tube and the filling in the transferring tube.
The monocrystal producing device of the invention is characterized in that the raw material melting tank is arranged above the crucible, and the raw material melt introducing means comprises a guide member for transferring, along its surface, the raw material melt that has leaked out and flown down from the fine hole made at the bottom portion of the raw material melting tank to guide the melt into the crucible.
According to the monocrystal producing device made as above, the raw material melt that has leaked out from the bottom portion of the raw material melting tank is transferred along the surface of the guide member, and is descended by the weight of itself, so as to be supplied into the crucible. Water or impurities remaining in the raw material melt are, before being put into the crucible, evaporated and removed by the heat from the electric furnace.
The monocrystal producing device of the invention is a monocrystal producing process for growing a crystal by arranging a crucible for melting a raw material in an electric furnace, keeping the crucible at a temperature not less than the melting point of the raw material, and pulling down and simultaneously rotating a seed crystal in the state that the upper end portion of the seed crystal is brought into contact with a raw material melt that has leaked out from a fine hole made at the bottom portion of the crucible, characterized by performing the crystal growth while supplying the raw material melt continuously into the crucible to keep the outflow amount of the raw material melt from the fine hole of the bottom portion of the crucible substantially constant by arranging a premelt inside or above the crucible inside the electric furnace, supplying continuously the powdery raw material in an appropriate amount at each time from the powdery raw material tank out of the electric furnace onto the premelt plate through a transferring tube so as to melt the powdery raw material on the premelt plate, and introducing the melt into a melt-collecting portion of the crucible.
According to the above-mentioned process, a monocrystal having a large diameter and a long size can easily be obtained. Moreover, the obtained crystal can have a stable composition since the process for growing the crystal from the powdery raw material is continuously performed.
The monocrystal producing process of the invention is a monocrystal producing process for growing a crystal by arranging a crucible for melting a raw material in an electric furnace, keeping the crucible at a temperature not less than the melting point of the raw material, and pulling down and simultaneously rotating a seed crystal in the state that the upper end portion of the seed crystal is brought into contact with a raw material melt that has leaked out from a fine hole made at the bottom portion of the crucible, characterized by performing the crystal growth while supplying the raw material melt continuously into the crucible to keep the outflow amount of the raw material melt from the hole of the bottom portion of the crucible substantially constant by arranging a raw material melting tank above the crucible inside the electric furnace, supplying the powdery raw material in an appropriate amount at each time from the powdery raw material tank out of the electric furnace onto the raw material melting tank through a transferring tube so as to melt the powdery raw material in the raw material melting tank, and subsequently introducing the melt into a melt-collecting portion of the crucible.
According to the above-mentioned process, a monocrystal having a large diameter and a long size can easily be obtained. Moreover, the obtained crystal can have a stable composition since the process for growing the crystal from the powdery raw material is continuously performed.
The monocrystal producing process of the invention is characterized in that the powdery raw material is a powdery raw material comprising a mixture of lithium (Li) powder and niobium (Nb) powder, and the component ratio of lithium to the total of lithium and niobium in the powdery raw material is from 48.5 to 50.0%.
According to the above-mentioned process, it is possible to produce a lithium niobate (LiNbO3) monocrystal having an incongruent melting composition wherein the component ratio of lithium to the total of lithium and niobium is from 48.5 to 50.0% and its diameter is 1.2 inches or more.
The monocrystal producing process of the invention is characterized in that the powdery raw material used in the process of claim 8 or 9 is a powdery raw material comprising a mixture of lithium (Li) powder and tantalum (Ta) powder, and the component ratio of lithium to the total of lithium and tantalum in the powdery raw material is from 48.5 to 50.0%.
According to the above-mentioned process, it is possible to produce a lithium tantalate (LiTaO3) monocrystal having an incongruent melting composition wherein the component ratio of lithium to the total of lithium and tantalum is from 48.5 to 50.0% and its diameter is 1.2 inches or more.
A monocrystal of the invention characterized in that it is a monocrystal having an incongruent melt composition and its diameter is 1.2 inches or more.
A monocrystal of the invention recited in claim 13 is characterized in that the monocrystal, having an incongruent melt component, is lithium niobate (LiNbO3) and the component ratio of lithium to the total of lithium and niobium contained therein is from 48.5 to 50.0%.
A monocrystal of the invention is characterized in that the monocrystal, having an incongruent melt component, is lithium tantalate (LiTaO3), and the component ratio of lithium to the total of lithium and tantalum contained therein is from 48.5 to 50.0%.
A monocrystal of the invention is characterized in that and the scattering in its Curie point is xc2x12xc2x0 C. or less.
A monocrystal of the invention is a wafer characterized by having characteristics and having a diameter of 1.2 inches or more.